The present application relates to a secondary battery electrode suitable for a lithium ion secondary battery and so on, a method for manufacturing the same, and a secondary battery employing the electrode, and mainly to improvement in the load characteristic.
In recent years, mobile apparatus having higher performance and more functions is being developed. This trend demands secondary batteries employed in the mobile apparatus as its power supply to have smaller size, lighter weight, smaller thickness, and higher capacity.
Among secondary batteries that can meet this demand is a lithium ion secondary battery. Battery characteristics of the lithium ion secondary battery greatly change depending on an employed electrode active material and so on. In a typical lithium ion secondary battery currently put into practical use, lithium cobalt oxide is used as its cathode active material and graphite is used as its anode active material. The battery capacity of the lithium ion secondary battery having such a configuration is approaching the theory capacity, and hence it is difficult to greatly increase the capacity through future improvements.
For a solution thereto, studies are being made on a scheme in which silicon or tin, which will be alloyed with lithium in charging, is used as the anode active material for realization of great increase in the capacity of the lithium ion secondary battery. However, when silicon or tin is used as the anode active material, the degree of the expansion and contraction of the anode active material layer accompanying charging and discharging is high, which leads to a problem of the lowering of the cycle characteristic due to the turning of the active material into fine particles or the separation of the active material from the anode current collector attributed to the expansion/contraction accompanying the charging/discharging.
As a related art, a coated-type anode obtained by applying slurry that contains a granular active material and a binder on an anode current collector has been used. In contrast, in recent years, there have been proposed anodes formed by depositing an anode active material layer such as a silicon layer on an anode current collector by a vapor-phase method, liquid-phase method, sintering, or the like (refer to e.g. Japanese Patent Laid-open No. Hei 8-50922, Japanese Patent No. 2948205, and Japanese Patent Laid-open No. Hei 11-135115). According to these documents, in such an anode, the anode active material layer is formed monolithically with the anode current collector. Therefore, compared with the coated-type anode, the fine-dividing of the active material due to expansion/contraction accompanying charging/discharging can be suppressed, and thus the initial discharge capacity and the charge/discharge cycle characteristic are improved. Furthermore, an advantage of improvement in the electron conductivity in the anode is also achieved.
However, also in this anode, for which an anode active material layer is formed monolithically with an anode current collector and the manufacturing method is improved, repetition of charging/discharging applies stress to the interface between the anode active material layer and the anode current collector due to the significant expansion/contraction of the anode active material layer, and thus the cycle characteristic will be lowered due to separation of the anode active material layer from the anode current collector, and so on.
To address this problem, in Japanese Patent Laid-open No. 2004-349162 (Pages 4, 5, and 8, FIGS. 2 and 3, hereinafter Patent Document 1), an anode composed of an anode current collector and an anode active material layer formed on the anode current collector by a vapor-phase method is proposed. This anode active material layer is formed by alternately stacking plural first layers and plural second layers that contain silicon and have different oxygen content. The first layers contain elemental silicon or a silicon alloy as the anode active material. Although the first layers may contain oxygen or may not, it is preferable that the oxygen content be low because lower oxygen content provides higher capacity. The second layers contain oxygen in addition to silicon. The oxygen is coupled with the silicon and thus exists as an oxide. It is preferable that in the second layers, the silicon content be 90 atomic % or lower and the oxygen content be 10 atomic % or higher.
According to Patent Document 1, in the anode having such a configuration, significant expansion/contraction accompanying charging/discharging is suppressed, and thus the structural destruction of the anode active material layer accompanying the charging/discharging is effectively suppressed. Furthermore, the reactivity between an electrolyte and the anode active material layer is reduced.
In the anode disclosed in Patent Document 1, the structural destruction of the anode active material layer is effectively suppressed, and thus the charge/discharge cycle characteristic is improved. However, the present inventors have found the following fact as a result of committed researches. Specifically, the oxygen-containing layer is merely one form of the active material layer having a multilayer structure, and further development in the active material layer having a multilayer structure can enhance not only the charge/discharge cycle characteristic but also the load characteristic.